The research proposed herein seeks to determine the structure and function of the iron-molybdenum cofactor (FeMoco) from the MoFe protein of Azotobacter vinelandii nitrogenase and how nitrogenase is organized to deliver the required reducing equivalents to substrate. Experiments include: purification and crystallization of FeMoco to determine its chemical composition and ultimately its structure by x-ray crystallography; x-ray absorption spectroscopy at Mo and S edges of FeMoco to gain structural insights and changes therein on redox and chemical modification; electrochemical, chemical and spectroscopic studies to elucidate FeMoco's redox properties and potential for catalysis; and the purification, characterization and reconstitution with FeMoco of the cofactor-deficient MoFe protein from mutant cells. This research aims to determine the individual responsibilities of FeMoco and its complementary protein in catalysis. Such information will be invaluable in attempts to enhance biological nitrogen fixation and to duplicate it synthetically.